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The zone of the body between the neck and the bottom of the ribs is known as the thorax. The major organs in the thorax are the heart, lungs and tongue. The lungs and associated airways allow us to breathe.

In the head the airways consist of the mouth and nasal passages. Air and food has a common passage in the throat.

The airways from the neck into the lungs have the following divisions:

Larynx or voicebox. This is where there is speech and sound generation.

Trachea or windpipe.

Two tubes that are each known as a bronchus, plural bronchi.

Bronchioles which are subdivisions of each bronchus.

Alveoli which are sacks at the end of the airways that allow oxygenation of the blood.

The key features of breathing are that when we breathe in the intercostal muscles between the ribs and the diaphragm both contract; when we breathe out both of these muscles relax. When we breathe in the contraction of the intercostals pulls the sternum up and away from the body and the descent of the diaphragm increases the volume of the thoracic cavity. Notice that in the resting state (breathing out) the diaphragm bulges up under the lungs, the lungs themselves are slightly elastic and pull the diaphragm back to this position

The illustrations below are an anatomically correct portrayal of the appearance of the lungs in a human being. The lung tissue itself appears pink in non-smoking country people and almost black in smokers who live in cities.

The lungs flank the heart and great vessels in the chest cavity. (Source: Gray's Anatomy of the Human Body, 20th ed. 1918.)

In this image, lung tissue has been dissected away to reveal the bronchioles. (Source: Gray's Anatomy of the Human Body, 20th ed. 1918.)

The two main functions of the lungs are to oxygenate the blood and to remove waste carbon dioxide.

The blood is oxygenated in the alveoli. The alveoli are thin walled and surrounded by capillaries. The blood enters the capillary network around the alveoli from the pulmonary artery and leaves the capillary network via the pulmonary vein.

Highly schematic diagram of the alveoli with both cross-section and external view

Oxygen diffuses into the blood through the alveolar and capillary walls and carbon dioxide diffuses out of the blood. The alveoli have a surface area of about 70 square metres to make this gas exchange as fast as possible.

Carbon dioxide dissolves in water and can easily and reversibly form compounds such as carbonic acid and bicarbonates.

Oxygen does not dissolve much in water, to overcome this problem the oxygen in the blood is stored in red blood cells. These contain haemoglobin which can combine with oxygen to form oxyhaemoglobin. The red blood cells contain the oxygen in the blood. The blood transports oxygen from the lungs to the rest of the body.

Oxy-haemoglobin is bright red and haemoglobin is dark red, this is why veins look dark and why all the diagrams show veins in blue and arteries in red. The exception is the pulmonary artery which carries dark red, de-oxygenated blood to the lungs and the pulmonary vein which carries bright red oxygenated blood away from the lungs.

The composition of exhaled air (air that is breathed out) is very different from the composition of inhaled air (air that is breathed in). Inhaled air has the same composition as normal air, it contains:

78% nitrogen

21% oxygen

1% inert gas such as argon

0.04% carbon dioxide

little water vapour

Exhaled air contains less oxygen and more carbon dioxide, it is also saturated with water vapour. Exhaled air contains:

78% nitrogen

16% oxygen

1% inert gas such as argon

4% carbon dioxide

saturated with water vapour

The difference between the amount of oxygen in inhaled and exhaled air is equal to the difference in the amount of carbon dioxide in exhaled and inhaled air.

Respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. Aerobic respiration uses oxygen to oxidise glucose and produce energy. The equation for the oxidation of glucose is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy released

Glucose + Oxygen -> Carbon dioxide + water + Energy released

In a fire there is a massive uncontrolled release of energy as light and heat. Respiration is a similar process but it occurs in gradual steps.

Most animals and plants use aerobic respiration as a primary source of energy.

Mitochondria are cell organelles which float around in the cytoplasm and are found in both plant and animal cells. In cells, aerobic respiration occurs in the mitochondria. Here, the energy is made in the form of a compound called ATP. However, some of the energy made is lost as heat energy. The ATP drives chemical reactions and is used by the cells as a source of energy to do this.

When a person is doing very heavy exercise and the blood cannot supply enough oxygen another sort of respiration occurs. This converts glucose into energy without the need for oxygen and is known as anaerobic respiration. The reaction is:

Glucose → Energy released + lactic acid

Anaerobic respiration releases less energy than aerobic respiration. Unfortunately the insufficient blood supply that leads to anaerobic respiration also means that the lactic acid builds up in the muscles. High lactic acid concentrations are painful and felt as cramp. When exercise stops, the blood supply is able to provide enough oxygen to convert the lactic acid to carbon dioxide and water but this takes time and the muscle pain may continue after exercise until the lactic acid has been converted.

The amount of Oxygen required to oxidize lactic acid that accumulates on muscles due to an aerobic respiration is known as the oxygen debt. Carbon dioxide and lactic acid both cause increases in breathing rate and heart rate to allow the body to repay the oxygen debt. The oxygen debt is the reason why we continue to be out of breath even after exercise. If athletes are very fit their circulation can provide extra oxygen more rapidly and their recovery time, the time required to restore normal breathing and pulse, will be shorter than in people who are not fit.

The direct conversion of glucose to energy without the use of oxygen occurs in many yeasts and fungi. The ethanol that is used in alcoholic drinks is a result of anaerobic respiration in yeast, the reaction is:

Glucose → Energy released + ethanol + carbon dioxide

Brewers use various types of brewers yeast to produce alcohol. In fizzy alcoholic drinks such as champagne the bottles are tightly stoppered to prevent the carbon dioxide from escaping.